Saturday, May 23, 2009

Dark Stars, Black Holes, Bright Galaxies

"Hearts of Darkness"


We live in a spiral galaxy. Our Solar System resides about three quarters of the way out from the centre of our Galaxy, or "Milky Way", in a spiral arm consisting of gas and young stars. However, galaxies exist in several different forms. Elliptical galaxies are large, round, aggregates of predominantly old stars. Spirals, like our Galaxy, possess disks with catherine wheel-like arms that are the sites of ongoing star formation.

An infrared image of our Galaxy taken by the Diffuse Infrared Background Experiment (DIRBE) instrument on the NASA Cosmic Background Explorer (COBE) satellite. The galactic plane runs horizontally along the middle of the image. Absorption by interstellar dust is minimized at infrared wavelengths allowing a clearer view of the plane and centre of our Galaxy.

Irregular galaxies, as their name implies, lack a well defined structure, but usually possess numerous star formation regions and large amounts of gas and interstellar dust (micron sized particles made up of carbon and silicon). Galaxies inhabit variously populated regions of space. The low density regions are well populated by spiral and irregular galaxies, whilst the denser, rich clusters are dominated by elliptical galaxies.

An image of Messier 87, a giant elliptical galaxy in the Virgo cluster.

It has become clear over the last 30 years that extremely dense objects exist both in our Galaxy and in the centres of many nearby galaxies. In our Galaxy (and most likely others) small regions of space weighing more than about 5 of our Suns exist. They consume nearby gas and stars and nothing ever escapes their grasp. In the centres of large galaxies similar regions of space exist that also consume stars and gas. However these regions can weigh as much as several billion (1 billion = 1,000,000,000 or 109) Suns.

This web site will describe the theory and observations of these black holes and recent observations of the centres of galaxies that are providing new ideas about galaxy structure and evolution. The galaxies with these exotic, extremely massive objects at their centres may well be called "Hearts of Darkness".

Dark Stars, Black Holes

Shine a torch upwards in the night sky. The light travels along a straight line then eventually fades, scattered by dust particles in the air. Travelling at 300,000 kilometres per second light is not hindered by the gravitational field of the Earth that requires an object to travel at least 11 kilometres per second to escape its influence. What mass would Earth need to be to stop the torch light from escaping? Based on Newton's gravitational laws the Earth would need a mass equivalent to 2100 times that of our Sun. Such a massive Earth would not be a very hospitable place to live! The intense gravitational field would crush pre-existing structures. If however we used the existing mass of Earth and could squeeze Earth into a sphere slightly smaller than a golf ball, again, light would not escape from its surface.

Theorists from the late 1930s onward predicted that small sized stellar objects could exist as the final products of stellar evolution. A "star" with a radius of 5 kilometres would need to weigh about 1.7 times the mass of the Sun to stop light escaping from its surface. Did such "dark" stars exist?

A schematic view of the formation of a neutron star. A supernova explosion leaves a massive core of neutrons behind.

The partial answer to this question was the discovery in 1967 of radio pulses that came from rotating neutron stars, or pulsars. Pulsars are extremely small, massive stars made of tightly packed neutrons. They are formed during a supernova explosion which occurs to high mass stars. Since their discovery, over one thousand pulsars in the Galaxy have been discovered. A New Zealand astronomer, Richard Manchester, who works at the Australian Telescope National Facility, is one of the worlds leading researchers of pulsars. Whilst neutron stars or pulsars are extremely massive and small, their largest escape velocity is still only about 80% of the speed of light. So they are close to being dark stars, but not quite!

People have been thinking about "dark stars" for over two centuries! In 1783 the Reverend John Michell delivered a paper to the Royal Society in London announcing that invisible stars may exist if they were massive enough. The Frenchman Pierre Laplace discussed a similar phenomenon several years later. Early this century the German astronomer Karl Schwarzschild succeeded in finding solutions to some outstanding problems in Einstein's theory of General Relativity, which describes gravity. Some solutions of Einstein's equations become infinite (called a singularity) at zero radius. Schwarzschild calculated that a singularity, could exist at a small radius for a very dense object. For the Sun this radius would be 3 kilometres. We know this radius nowadays as the Schwarzschild (or gravitational) radius, and it is that required by an object so that radiation cannot escape from it. In 1933 astronomers Walter Baade and Fritz Zwicky suggested that the remnant of a supernova explosion could be a very dense star composed of neutrons.

An artist's impression of a supernova, the explosion of a star.

In 1939 Robert Oppenheimer and colleagues used quantum theory to determine that stable neutron stars could exist, and then went further, publishing a paper that would become a classic. It described massive stars that, once finished thermonuclear burning, would collapse forever. A physical model for a "dark star" had been found!

A photograph of Supernova 1987A (the bright star lower, right) next to the Tarantula Nebula in the Large Magellanic Cloud. This was taken by Alan Gilmore on the 8th of March 1987 using the 60cm reflector at Mount John University Observatory, Lake Tekapo. The image has been inverted so that bright features appear dark.

Let's stop for a moment. A problem is looming! How would you detect an object whose gravitational field is so great that all radiation (light emitted from a torch is just one type of radiation) cannot escape from it? The answer is that you cannot observe it directly, but possibly indirectly, by observing its effect on surrounding objects.

As it turns out, any star greater than 3 solar masses must eventually form such a "dark star" after thermonuclear reactions have ceased, since no known source of pressure can support it. These objects are called "black holes" and this term was first coined by the physicist John Wheeler.

In 1963 a New Zealand mathematician, Roy Kerr, then working at the University of Texas, found solutions to the general relativistic field equations for the case of a rotating star. Since stars rotate, black holes should rotate, and these solutions were critical in understanding the space-time effects of spinning black holes. A major breakthrough had been made. Kerrs solutions showed that as well as having an event horizon (at the gravitational radius) a spinning black hole had another important horizon, at a greater radius than the event horizon, called the static limit. The region between the event horizon and the static limit is called the ergosphere. Later studies by Penrose, Wheeler, Bekenstein and Hawking amazingly showed that black holes could emit radiation from the ergosphere. In general, the smaller a black hole, the larger the amount of radiation could be emitted. However, even for stellar mass black holes the rate of radiation is very small, so that they exist for hundreds of billions of years.

So, are there any black hole candidates? Yes, there exists strong, indirect, evidence for many. One observational signature is the rapid variation of high energy X-rays from an object. This variation can be caused by a binary star system that consists of a black hole orbiting a very large (supergiant) star. Gas from the supergiant is gravitationally attracted to the black hole and as the gas approaches it heats up to 1 million degrees and emits high energy X-rays. A decrease in the strength of X-rays from the binary system is explained when the black hole goes behind the supergiant during its orbit. Many such binary systems are known. One system, Cygnus X-1, in the northern sky constellation Cygnus, is one of the best candidates for a black hole.

Artist's impression of the Cygnus X-1 binary system, with the supergiant star on the left, and the black hole surrounded by an accretion disk of gas, on the right.

Another strong candidate for a black hole is LMC X-1. LMC stands for Large Magellanic Cloud, a close neighbour galaxy to our Galaxy. LMC X-1 is the strongest source of X-rays in the LMC and it originates from an unusually energetic binary star system. This source is thought to be a normal and compact star orbiting each other, similar to the Cygnus X-1 system. The X-rays shining from the system knock electrons off atoms, causing some atoms to glow noticeably in X-rays. Motion in the binary system indicates the compact star is probably a black hole, since its high mass - roughly five times that of our Sun - should be massive enough to cause even a neutron star to collapse.

An X-ray image of LMC X-1 taken with Röntgensatellit (ROSAT).

Active Galaxies and Central Energy Sources

Many galaxies possess nuclei that emit vast amounts of radiation. The amounts can vary from a small fraction to several thousand times greater than the radiation output of an entire normal host galaxy. In the 1950s and 1960s radio astronomy provided important clues to the nature of such galaxies. Powerful radio sources in the sky were found to be associated with faint elliptical galaxies. Many showed dual lobes of radio emission on opposite sides of the optical galaxy. The radio emission was caused by radiation from high velocity, spiralling electrons in strong magnetic fields. This radiation is called synchrotron radiation. It was quickly realised that the majority of the radiation from such galaxies (called active) was not from stellar sources, but due to this type of high velocity particle emission.

A schematic illustration of synchrotron radiation. Electrons spiral around magnetic field lines emitting photons of radiation.

Some clues indicated the probable extreme power source of activity in galaxies. The radio lobes observed on either side of the optical galaxy were sometimes connected to a small, emission region in the nucleus of the galaxy via narrow, straight jets. Energy arguments suggested that the lobes of emission had to be continually replenished by fast moving electrons. The presence of jets joining the nucleus to the lobes suggested that something in the small nucleus was the energy source. Variability in the optical and radio emission of the nucleus on time scales of hours also suggested a very small energy producing region (of light hours diameter, similar in size to the Solar System).

Cygnus A: An image obtained with the Very Large Array (VLA) radio telescope in New Mexico at a wavelength of 6 centimetres. Note the bright lobes, and narrow jets that point back to the nucleus. The optical galaxy lies well within the radio lobes, centred on the radio nucleus.

It is now generally believed that such activity in galaxies is powered by supermassive objects in their nuclei.

Supermassive objects or black holes?

The presence of supermassive objects in galaxy centres was first inferred in the late 1970s. Imaging and spectral observations of the nucleus of the large elliptical galaxy in the Virgo cluster of galaxies, Messier 87 (or M87, see image above), by Peter Young and Wallace Sargent and collaborators, suggested the existence of a compact object of 5 billion solar masses within 300 light years of the nucleus. This amount of mass is difficult to explain by normal populations of stars, and many astronomers were convinced that supermassive black holes (SBHs) easily explained the observations.

Further, the very small size and enormous energy outputs of these nuclear regions strongly suggest black hole accretion (mass converted to energy by the extreme gravitational field of the black hole) as the energy source. Rapid progress has been made recently in the study of central regions of galaxies by using the high resolution capabilities of the Hubble Space Telescope (HST) and radio telescopes on Earth. HST is in orbit around the Earth, and is above the atmosphere that blurs ground-based optical telescope images.

HST above the Space Shuttle. The gold panels are solar arrays used to power the telescope. The central white rectangle is the cover of the Wide Field Planetary Camera 2 instrument that has taken many high resolution images of galaxy nuclei.

A matter of perspective? The Unified Model

It is now apparent that many features of active galaxies are common. A model has been put forward that tries to reconcile the differing properties of activity by assuming that the physical structure in the nucleus of all active galaxies is similar. The "unified model" assumes that all active galaxies possess a SBH surrounded by dust in the shape of a torus (doughnut-like). Relativistic jets (ie. radio jets) if detected will appear at right angles to the major axis of the torus.

Variations to the model include the evolutionary status of the SBH (eg. its mass, possible spin), the type of host galaxy (ie. spiral or elliptical), the accretion rate of fuel (ie. gas, stars) into the nuclear (accretion disk + SBH) region, and importantly, the aspect or orientation of the torus to our line of sight. Such model variations go a long way to explain the variety of physical properties seen in active galaxies.

Schematic diagram, not to scale, of the central region of a Seyfert galaxy illustrating the effect of viewing angle. HBLR/BLR stands for Hidden/Broad Line Region (high velocity gas) close to the nucleus, NLR is Narrow Line Region (low velocity gas). Broad spectral lines are produced by gas clouds with large internal velocities.

By looking along a line of sight into the hole of the torus, we see the highest velocity gas clouds, nearest to the SBH. Such galaxies are classified as Seyfert 1, Quasar and Blazar. If the torus obstructs our direct view, we can only observe lower velocity gas clouds, further from the SBH, and possibly scattered light from the nuclear region, and we then detect active galaxies of the Seyfert 2 and radio galaxy types. In rough order of increasing luminosity the active galaxies are Seyferts, Radio Galaxies, Blazars and Quasars. It is now thought that the host galaxies of Seyferts are spirals, and elliptical galaxies host radio galaxies and quasars although there could be some overlap. Also, many distant quasars imaged by HST show peculiar structures that are indicative of interacting or merging galaxies, suggesting that collisions between galaxies may help to produce the high luminosity quasars.

An artist's impression, based on HST observations, of a warped, dusty disk around a suspected SBH in NGC 6251. Perpendicular to the disk is a jet of relativistic particles ejected along the SBH spin axis.

Nearby Monsters

NGC 4261 - A large, dusty disk

NGC 4261 is a bright elliptical galaxy. It has radio jets extending well outside the optical galaxy. The HST image shows a large, about 400 light years in diameter, dusty disk slightly inclined to our line of sight. Note that the radio jets are aligned perpendicularly to the major axis of the dusty disk (ie. the extended cool region of a torus) consistent with the unified model. HST spectral observations of gas in the nucleus suggest a 5 x 108 solar mass SBH.

NGC 4261, Left: A ground based composite optical (white) and radio (yellow/orange) image. Right: HST image of the galaxy centre showing the disk of dust. Interestingly, the suspected SBH is some 20 light years from the geometrical centre of the galaxy. The reason for this misalignment is unknown.

A word of warning. Even though HST allows us the clearest optical view of galaxy centres, we do not directly resolve the SBHs or their gaseous accretion disks. For example, NGC 4261 is approximately 82 million light years distant, and at that distance, an SBH accretion disk of 1 light week diameter would span about 1/1000 the size of a HST imaging pixel element. What we do see in HST images however are the cooler, dusty disks surrounding the SBH and hot accretion disk. However, the resolving power of HST does allow important velocity measurements at small distances from the nucleus, which constrains the mass contained within that distance.

Messier 87 - revisited

M87 is one of the nearest ellipticals that shows signs of activity. As long ago as 1918 H. D. Curtis discovered an optical "jet" originating from the nucleus. The optical emission from the jet is also synchrotron radiation, seen usually as radio emission. The synchrotron jet occurs at optical wavelengths when the fast moving electrons are very energetic. M87 is a powerful radio source (known as 3C 274 and Virgo A) and the radio source at the nucleus is compact, spanning a diameter of less than 3 light-months.

M87 as observed by HST showing the nuclear gas disk (lower left) and jet.

HST detects a small disk of gas in the nucleus. The disk is approximately elliptical in shape, and its minor axis is close to the direction of the optical synchrotron jet. Radial velocity measurements along the gas disk shows high recession and approach velocities of 500 kilometres per second. A central mass of 2 billion solar masses is deduced. The authors conclude that the disk of gas is feeding a SBH in the nucleus, consistent with (but smaller than, by a factor of about two) the mass inferred from the measurements in the 1970s mentioned previously.

HST optical observations of M87, showing the nuclear gas disk, and the spectral signature of rotation. A gas emission line from two regions of the disk shows a shift in wavelength indicative of very high relative velocities.

A Mini-Monster in our backyard!

For a number of years evidence has been growing that the centre of our Galaxy may harbour a SBH. The motions of stars around our Galaxy centre indicate increased velocities down to very small distances, about 10 light days. The density of matter needed to explain such motions rules out most alternatives to a SBH.

Left: A near-infrared image of the central 3 light years of the Galaxy centre. The observation was made with the SHARP I camera on the NTT telescope at ESO, La Silla, Chile. Right: A contour plot of the image. The compact radio source Sgr A*, which is associated with a 3 million solar mass black hole, is just above the central label "SW".

A radio image of the Galactic Centre at a wavelength of 6 cm, taken with the VLA. The region is known as Sgr A West (encompassing Sgr A*) and the emission is due to gas being heated by nearby hot, young stars.

As in the case of stellar mass black hole systems, we may expect to detect large amounts of X-rays from an accretion disk around a Galactic Centre SBH. However, observations have resolved most of the X-ray emission in the region to a handful of unrelated X-ray binary systems. The X-ray luminosity of the Galactic Centre is some 7 orders of magnitude lower than expected for an accretion disk around a 3 million solar mass SBH. It is therefore possible that if a SBH does reside in the centre of our Galaxy, it is dormant.

Where to now?

The picture that has emerged is as follows. SBHs are probably a normal feature of the central regions of bright galaxies that have spheroidal components (eg. elliptical galaxies, spiral galaxies with a bulges). SBHs have not been detected in irregular galaxies. The SBH masses scale roughly with the mass of the host galaxy, implying a strong link between the growth of the galaxy as a whole, and the growth of the SBH.

Some fundamental questions remain however. What is the link between SBHs seen today in relatively nearby and lower luminosity galaxies to distant, very luminous quasars? Quasars were more populous in the early universe, and so it is possible that many nearby galaxies were quasars in their youth, and now harbour relic SBHs that earlier emitted high (quasar) luminosities. How do SBHs evolve? We also believe that galaxy mergers were more prevalent at earlier times in the Universe. What part then do galaxy mergers play in SBH evolution? How would two pre-existing SBHs behave if their host galaxies merged? Such events may not be observable by the usual optical, radio or X-ray telescopes, but by the detection of gravitational waves. A merger of two 107 solar mass SBHs would radiate energy at a frequency of about 10-4 Hz.

The European Space Agency (ESA) is planning a space-based gravity wave detector, called Laser Interferometric Space Array (LISA). The primary objective of the LISA mission is to detect and observe gravitational waves from massive black holes and galactic binary stars in the frequency range 10-4 to 10-1 Hz. Useful measurements in this frequency range cannot be made on the ground because of the unshieldable background of local gravitational noise. From recent research and upcoming missions like LISA we are finally shining some light on these enigmatic hearts of darkness.

An artist's impression of LISA. It consists of six identical spacecraft forming an equilateral triangle in space with two closely spaced (200 kilometres) "near" spacecraft at each vertex. When a gravity wave passes through the system it causes a strain distortion of space which will be detected by measuring the fluctuations in separation between proof masses inside the spacecraft.

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